If your shotgun is like mine there should be a metal plate about half way up the wood stock. In that plat will be a decorative bar - that bar should slide out to the left of the right, thus detaching the wood stock from the barrell.
I found mine under the barrell. You will need to remove the wood under the barrell in front of the triggers.
Many manufacturers manufacture double barrel shotguns in either a side by side (SxS) or Over/Under configuration. double barrel was made on July 1 1875 in England. by westley Richards co. it is a hammerless 12 guage
This is a double barrel, over/under shotgun.
There should be a screw or release lever under the forearm of the gun. Once you release that you should be able to remove the barrel.
Check the receiver, barrel(s), under the barrel(s), under butt place.
proof mark
Whichever one you pull the trigger to.
A weapon that has one shotgun barrel and one rifle barrel either in an over/under or side by side configuration.
You may have to break the shotgun down in to the barrel section and the receiver section first.Look under the barrel section on the flat just ahead of the lug for the serial number.
Best left to a gunsmith.
Single barrel, under $100. Double-barrel up to $200 unless heavily ornamented.
No way to answer without knowing barrel length and gauge.